The invention relates to the acquisition and processing of seismic data obtained using transmitter of seismic waves and receivers of down-going or up-going seismic waves, some of which are arranged on the surface near to an exploratory well and the others in the exploratory well at a succession of measurement points.
It is possible either to install a transmitter at the surface and lower a receiver down the well, or to install a receiver at the surface and lower a transmitter down the well. In general, a single piece of equipment is lowered down the well and fixed to the walls of the well by means of a mobile arm at a succession of positions which constitute the measurement points.
All the methods in current use require the provision, within the exploratory well, of a number of measurement points which are separated from each other by a distance less than a predetermined limit distance in order to avoid the ambiguity known as "aliasing" in the dephasing of the seismic waves received by the receiving equipment. This leads to a high cost, since seismic operations require a large staff and large quantities of equipment and are thus very time-consuming. Moreover, the desired precision is not always achieved, since, in determining the limit distance, all the waves and in particular the wave known as the tube wave are not taken into account, so as to avoid using an excessive number of measurment points.
The terms "down-going" and "up-going" as applied to seismic waves are used herein to describe waves the paths of which appear to go down and up, respectively, if the wave is considered as arriving at the measurement point. In other words, where a receiver is located at the measurement point, a down-going wave is one which travels downwardly to the measurement point and an up-going wave is one which travels upwardly to the measurement point. Where a transmitter is located at the measurement point, a down-going wave is one which travels upwardly from the measurement point and an up-going wave is one which travels downwardly from the measurement point.
With a view to reducing the number of measurement points necessary for the same quality of measurement, and to improving the latitude in the distribution of the measurement points so that certain zones may be favoured and a greater degree of precision can be obtained than obtained hitherto, an analysis was first made of the reasons for the restrictive nature of the known methods. It was found that the error in these methods lay in placing stress on coherence filtering and subordinating the separation of the descending and rising waves to this filtering, these two operations being carried out together; this leads, in particular, to an increased number of measurement points without any appreciable improvement in the quality of the measurement.